碳正离子Carbocation

带正电的碳原子为 sp² 杂化，三个取代基与它处于同一平面，键角约 120°；垂直于该平面有一个空的 p 轨道。亲核试剂可以从平面两侧进攻这个空轨道，所以由手性底物经碳正离子得到的产物通常是外消旋混合物。

The positive carbon is sp² hybridised, with its three substituents in one plane at about 120°, and an empty p orbital perpendicular to that plane. Nucleophiles can attack the empty orbital from either face, so a chiral substrate reacting through a carbocation usually gives a racemic product.

稳定性顺序为：叔 > 仲 > 伯 > 甲基碳正离子。烷基通过诱导效应向正电中心推电子，相邻 C–H 键的超共轭也能分散正电荷，烷基越多正电荷越分散，离子越稳定。苄基和烯丙基碳正离子可通过共轭使正电荷离域，也比较稳定。

The order of stability is tertiary > secondary > primary > methyl. Alkyl groups push electron density towards the positive centre by the inductive effect, and hyperconjugation with neighbouring C–H bonds also spreads the charge; the more alkyl groups, the more the charge is spread and the more stable the ion. Benzylic and allylic carbocations are also relatively stable because the charge is delocalised by conjugation.

HBr 与不对称烯烃加成时，H⁺ 加到含氢较多的双键碳上，生成更稳定的碳正离子，这就是马尔科夫尼科夫规则的原因。叔卤代烷水解按 SN1 机理进行，第一步生成叔碳正离子，是决速步。碳正离子还会通过氢迁移或烷基迁移重排为更稳定的碳正离子。

When HBr adds to an unsymmetrical alkene, H⁺ adds to the double-bond carbon bearing more hydrogens so as to give the more stable carbocation; this is the reason for Markovnikov's rule. Tertiary halogenoalkanes hydrolyse by the SN1 mechanism, whose first, rate-determining step forms a tertiary carbocation. Carbocations can also rearrange to more stable ones by hydride or alkyl shifts.

旧称 carbonium ion，现在 IUPAC 把三配位的 R₃C⁺ 称为 carbenium ion，把五配位的 CH₅⁺ 一类称为 carbonium ion，carbocation 为两者的总称。二十世纪六十年代，欧拉用 SbF₅–FSO₃H 等超强酸使碳正离子在低温下长期存在，并用核磁共振直接观测到它们。

The older name was carbonium ion; IUPAC now calls tricoordinate R₃C⁺ a carbenium ion and pentacoordinate species such as CH₅⁺ carbonium ions, with carbocation covering both. In the 1960s Olah used superacids such as SbF₅–FSO₃H to keep carbocations alive at low temperature and observed them directly by NMR.